An LED is a semiconductor device which converts electrical energy into light using characteristics of a semiconductor including specific compounds. The LED has various advantages, such as very small power consumption is due to high light conversion efficiency, appropriateness for miniaturization, slimming and weight reduction and unlimited applicability due to its small light source, semi-permanent and long lifespan (a blue, violet, or UV LED has a lifespan of about 100,000 hours, and a white LED has a lifespan of about 30,000 hours), very high response speed due to no need for pre-heating by elimination of the use of thermoluminescence or electroluminescence, a very simple lighting circuit, high impact resistance, safety and few environmental pollution factors due to no use of discharge gas and no filament, pulse operation at high repetition rate, reduction in visual fatigue, and realization of full color. Accordingly, the LED is widely used for light sources for liquid crystal display (LCD) backlights of mobile phones, camcorders, digital cameras, personal digital assistants (PDAs), etc., traffic lamps, electronic display boards, car headlights/taillights, display lamps of various kinds of electronic devices, office machines, facsimiles, etc., night lighting of remote controllers or surveillance cameras, infrared communication devices, information displays of outdoor advertising boards using various combinations of RGB pixels, ultra-precision displays of electronic display boards, and high-efficiency indoor/outdoor lighting. Particularly, as a high-brightness LED solving general problems of a conventional LED such as low brightness is commercially available, the use and application of the high-brightness LED have been rapidly expanded.
Particularly, since a white LED is very useful as a light source for an LCD backlight and indoor/outdoor lighting, usage has thereof rapidly increased. Also, just as fluorescent lamps drove incandescent lamps out of the market, it is expected that LED lamps will drive fluorescent lamps from the market.
A method for obtaining white light by an LED will be described hereinafter.
First, in a classical method for obtaining white light, three types of LEDs, that is, a red LED, a green LED, and a blue LED, are combined to obtain white light. However, this method has problems in that it requires a relatively high manufacturing cost, increases product size due to a complicated operating circuitry, and provides low optical characteristics and reliability of the product due to difference in temperature characteristics of the three LEDs, and thus is not substantially used at present.
Recently, in another method for obtaining white light, a white LED is selected as a single LED for generating white light. In this method, the surface of the white LED is coated with a phosphor, or the periphery of the LED or a lens is molded together with the phosphor such that the phosphor can be excited by light emitted from the LED and having a specific wavelength to generate light having a different wavelength. Then, the generated light is mixed with the light emitted from the single LED chip to generate white light.
However, in such a conventional method, the surface of a blue, violet or UV LED is directly coated with a phosphor, or the periphery of the LED or the lens is molded together with the phosphor. Thus, this method has a problem in that the lifespan of the LED is significantly reduced to one third or less due to LED degradation resulting from deterioration in heat dissipation. Particularly, when the phosphor is not evenly coated or dispersed, luminescent colors becomes non-uniform. However, it is very difficult to achieve uniform coating or dispersion/distribution of the phosphor.
As one of the most widely used white LEDs, U.S. Pat. No. 5,998,925 (Nichia Corp.) discloses a white LED, in which an InGaN-based blue LED emitting blue light having a wavelength of 450 nm is coated or molded with a yellow phosphor (generally, yttrium-aluminum-garnet:Y3Al5O12:Ce, YAG-based compound) such that blue light emitted from the blue LED excites the YAG yellow phosphor to emit yellow light in a wide peak, thereby allowing light components in two different wavelength bands, that is, the narrow-peak blue light of the blue LED and the wide-peak yellow light of the YAG-based yellow phosphor, to be recognized as white light by human's eyes a viewer through complementary interference.
However, the white light results from a mixture of the light components, which have different wavelengths and are not in a complete completely complementary relationship, and thus has only part of a visible range spectrum. For this reason, the white light has a color rendering index (CRI) of about 60˜75, and is generally not accepted as near-natural white light. Thus, it does not satisfy requirements for general indoor lighting. Also, the white LED has a problem of low brightness, because the blue LED shows the highest efficiency by excitation light at a wavelength of about 405 nm whereas the YAG-based phosphor is excited by blue light in a wavelength band of 450˜460 nm. Particularly, in coating or molding of the YAG-based phosphor, it is difficult to guarantee homogeneous and uniform dispersibility, thereby deteriorating uniformity and reproducibility of products in terms of brightness and spectral distribution of white light, and significantly reducing the lifespan of the LED.
In order to overcome the problems of the white LED including the blue LED and the YAG-based phosphor, U.S. Pat. No. 5,952,681 (Solidlite Corp.) discloses a technology for obtaining three-wavelength, high-CRI and near-natural white light by combining red, green and blue phosphors, and using a high brightness UV LED, which emits light in a wavelength band of 250 nm to 390 nm as an excitation light source. However, the use of the white LED has a problem in that the blue and green phosphors have satisfactory light emission efficiency while the red phosphor has low light emission efficiency. Particularly, the UV LED tends to deteriorate an organic resin by UV having a strong energy, thereby significantly reducing the lifespan of the LED.
In another type of white LED (Solidlite Corp.), a violet LED emitting light in a wavelength band of 390 nm to 410 nm is used and white light is obtained by combining red, blue, and green phosphors. The high brightness violet LED is commercially available from Cree Corporation (U.S.), and is known to emit a relatively natural three-wavelength band white light through uniform light emission from red, blue, and green phosphors excited by violet light in a wavelength band of 390 nm to 410 nm.
Factors affecting the characteristics of white light emitted from a white LED may include the intensity of the light, combination applicability of the light emitted from the LED and light converted by a phosphor, and the components, content and dispersed state of the phosphor. These factors have a significant influence on the emitted light. Particularly, white light emitted by combination of the blue LED and the YAG-based phosphor may have a problem in that the emitted color is generally biased to blue or yellow color due to difficulty in adjustment of the amount of a yellow phosphor and uniform dispersion thereof.
In order to obtain a white LED having excellent luminescent characteristics, it is necessary for a phosphor to be evenly dispersed in a light-transmitting matrix resin. However, in a fabrication process, before the matrix resin is completely hardened, a phosphor having a much higher specific gravity (the phosphor has a specific gravity of about 3.8˜6.0, although it depends on the kind of the phosphor) than the matrix resin is precipitated in a lower region of the light-transmitting matrix resin having a low specific gravity (for example, an epoxy resin has a specific gravity of about 1.1˜1.5), thereby making it difficult to obtain white light having excellent luminescent characteristics. Furthermore, it is not easy to precisely control the degree of dispersion of the phosphor. Accordingly, it is not easy to fabricate a high-quality white LED device and fabrication reproducibility is not good.
Meanwhile, an LED lighting device includes an LED lens, which allows light components diffused and emitted from an LED upon application of voltage to be directed as parallel light beams and can increase the intensity of radiation through a viewing angle. In addition, the viewing angle is adjusted by controlling curvatures of a light-incident lower surface and a light-emitting upper surface of the lens, and the lens can be suitably selected and used according to various shapes and sizes of lenses based on various parameters, such as the kind and power of a used LED, use purpose, an end user preference, desired intensity of lighting, and the like.
FIG. 1 is a sectional view of a conventional LED lens. The conventional LED lens generally has a hemispherical shape with a wide upper section and a narrow lower section, without being limited thereto. The LED lens includes an upper surface 7a having an annular lateral portion 7 and a flange 8, and is formed on a lower surface thereof with a cylindrical LED mounting portion 9. The LED mounting portion 9 may have a flat shape, but is generally formed with an internally convex portion 12 for collection of light.
The upper surface 7a of the LED lens may have a pectination shape, a plurality of dots, or a smooth planar shape in order to provide soft illumination. Also, the upper surface of the LED lens may have an opening at the center thereof. The lateral portion 7 may have various angle-gradients and lengths for adjustment of an irradiation angle. Further, the upper surface 7a may be formed into a forwardly projecting convex shape, a flat shape, a concave shape, or other specific shapes.
Reference numeral 5 denotes a substrate and reference numeral 6 denotes a light diffusing lens for LED molding.
FIG. 2 is an exploded perspective view of a conventional edge-type backlight unit. The conventional backlight unit 100′ generally includes a light source 15a, a light guide plate 10′ having one end facing the light source 15a, a reflective sheet 20 disposed below the light guide plate 10′, a prism sheet 30 disposed above the light guide plate 10′, a light diffusing sheet 40 disposed above the prism sheet 30, and a protective sheet 50 disposed above the light diffusing sheet 40.
More specifically, a light source 15 includes a linear light source 15a or a white LED (not shown) and a reflective plate 15b, and is located adjacent to a thick side surface of the light guide plate 10′ that generally has a tapered shape. The reflective sheet 20 is located below the light guide plate 10′, and the prism sheet 30, the light diffusing sheet 40 and the protective sheet 50 are sequentially stacked on an upper surface of the light guide plate 10′. The prism sheet 13 has a pattern of plural prisms (not shown) parallel to each other.
The light guide plate 10′ is formed with a light exiting surface 11 on an upper surface thereof and has a lower surface 13 adjoining the reflective sheet 20. A flat light entering surface 12 is formed on one side surface of the light guide plate adjacent to the light source 15, and the lower surface 13 of the light guide plate 10′ is formed with a pattern of plural prisms 14 each having prism slopes 14a, 14b and parallel to each other in a direction orthogonal to a traveling direction of light emitted from the light source 15.
Here, light emitted from the light source 15 is received by the flat light entering surface 12 and is scattered by the prism slopes 14a, 14b of the prisms 14 under the light guide plate 10′. Then, the light is emitted toward the prism sheet 30 through the light exiting surface 11 of the light guide plate 10′ and is scattered again by the prism sheet 30 having the pattern of plural prisms (not shown) orthogonal to the pattern of prisms 14, which is formed on the lower surface 13 of the light guide plate 10′. Then, the light is converted into uniform light and output through the light diffusing sheet 40.
Since the light diffusing sheet 40 serves to convert incident light into uniform light over the entire area of a display panel through diffusion and scattering, stacking the light diffusing sheet 40 on the prism sheet 30 makes it difficult to reduce the thickness of the backlight unit and increases the number of processes and components, causing deterioration in economic feasibility and process efficiency.